It is a well-known fact that lithium ion secondary batteries, which have high energy density and whose discharge capacity does not significantly decrease, have been used as a power source for mobile tools such as mobile phones and laptop computers. In recent years, with the miniaturization of mobile tools, there also is a demand for the miniaturization of lithium ion secondary batteries to be mounted therein. In addition, with the development of hybrid cars, solar power generation, and other technologies as a measure to prevent global warming, etc., new applications of supercapacitors having high energy density, such as electrical double-layer capacitors, redox capacitors, and lithium ion capacitors, have been increasingly expanding, and there is a demand for a further increase in their energy density.
An electrical storage device such as a lithium ion secondary battery or a supercapacitor, has a structure in which, for example, a positive electrode and a negative electrode are arranged via a separator made of polyolefin or the like in an organic electrolytic solution containing a fluorine-containing compound such as LiPF6 or NR4.BF4 (R is an alkyl group) as an electrolyte. Generally, the positive electrode includes a positive electrode active material, such as LiCoO2 (lithium cobalt oxide) or active carbon, and a positive electrode current collector, while the negative electrode includes a negative electrode active material, such as graphite or active carbon, and a negative electrode current collector. With respect to their shape, generally, the active material is applied to the surface of the current collector and formed into a sheet. The electrodes are each subjected to high voltage and also immersed in the organic electrolytic solution that contains a fluorine-containing compound, which is highly corrosive. Accordingly, in particular, materials for a positive electrode current collector are required to have excellent electrical conductivity together with excellent corrosion resistance. Under such circumstances, currently, aluminum, which is a good electrical conductor and forms a passive film on the surface to have excellent corrosion resistance, is almost 100% used as a material for a positive electrode current collector. Incidentally, as materials for a negative electrode current collector, copper, nickel, and the like can be mentioned.
One method for providing an electrical storage device with smaller size and higher energy density is to thin a current collector that constitutes a sheet-shaped electrode. Currently, an aluminum foil having a thickness of about 15 to 20 μm produced by rolling is generally used as a positive electrode current collector. Therefore, the object can be achieved by further reducing the thickness of such an aluminum foil. However, with rolling, further reduction of the foil thickness on an industrial production scale is difficult.
Thus, as an aluminum foil production method to replace rolling, a method that produces an aluminum foil by electrolysis, that is, a method that produces an electrolytic aluminum foil, has been attracting attention. In Patent Document 1, the research group of the present inventors has proposed a method for producing an electrolytic aluminum foil, comprising forming an aluminum film on the surface of a substrate by electrolysis using a plating solution containing at least a dialkyl sulfone, an aluminum halide, and a nitrogen-containing compound, and then separating the film from the substrate.
In the case of an industrial-scale electrolytic aluminum foil production, it is preferable that the step of forming an aluminum film on the surface of a substrate and the step of separating the film from the substrate are performed continuously using a cathode drum, rather than batchwise. The production of an electrolytic aluminum foil using a cathode drum comprises, for example, applying a current between a cathode drum partially immersed in a plating solution and an anode plate immersed in the plating solution to form an aluminum film on the surface of the cathode drum, and then separating, from the cathode drum, the aluminum film raised from the liquid surface by rotating the cathode drum. Such production can be performed using an electrolytic aluminum foil production apparatus as described in Patent Document 2. The aluminum film separated from the cathode drum can be, as an electrolytic aluminum foil, washed with water, then dried, and used for various applications.